Weed control in sunflower should begin well before the crop is even planted

The most serious losses from weed competition in sunflower occur during the first four weeks after crop emergence, according to Leon Wrage, South Dakota State University extension weed specialist.

Needless to say, your weed control management should begin well before then. This spring, avoid planting sunflower on fields with a heavy weed history or where perennials are a problem. Note too that carryover of herbicides used the previous season may injure sunflower.

Wrage is a strong believer in a chemical “burndown” of weeds in sunflower before planting even begins. “Weeds just absolutely have to be controlled at the starting point,” he says. “Experiments and experience suggests that split treatments of glyphosate is perhaps the best early-season approach for the sunflower producer.”

He advises the first application three to four weeks before planting to control the season’s first flush of weeds, including small wild buckwheat at the two to three-leaf stage. Follow up with another application at planting to get additional weeds.

“One problem with performance is where they have been trying to do a burndown at planting when weeds have already grown large,” Wrage says. “But when weeds get too big it can be difficult to get all of the axillary buds along the stem killed, and you may get regrowth or an incomplete kill. Doing a burndown when weeds are small before planting sunflower and then another at planting, you should be able to get control in the 90s (percent).”

Keep in mind that temperature and moisture stress can affect herbicide performance, including glyphosate, which depends on translocation. This fact was evident last year. “When it was cold early, weed growth in some cases was so slow it took two weeks before you could notice results from that first early burndown,” Wrage says. “When there was moisture stress, some weeds, kochia for example, just stopped growing. Then when we got moisture, treatment effectiveness picked up again.”

Last year’s drought could mean more weed worries for sunflower growers this year. “There were a lot of wheat fields that were hayed and didn’t get a later treatment. Huge seed crops were produced. There was a lot of kochia and Russian thistle that rolled into fences. Fields that didn’t have kochia before might have scattered seed now, and weeds from other adjacent areas as well.”

Spartan is the most effective option currently available for kochia control in no-till. Consider applying it at least two weeks before planting to allow ample time for moisture to trigger product activation. “Then follow up with that burndown at planting so kochia that emerged during that period can be burned off,” says Wrage.

Generally, about one-half to three-quarters of an inch of moisture is needed for treatment activation. Sam Tutt, northern technical manager for FMC, manufacturer of Spartan, says the product can be applied up to 30 days prior to planting without jeopardizing treatment effectiveness.

Trifluralin (Treflan) and Sonalan offer grass and some broadleaf weed control; and Prowl offers early preplant control of foxtail. But the pre-plant treatments need incorporation, which risks disturbing precious topsoil moisture. A light harrow is not adequate for the incorporation needed to work granules into the two inches or so of soil for optimum weed control, says Wrage.

The need for good sprayer cleanout when switching from one chemical to another is obvious. Sunflower is highly sensitive to herbicides such as 2,4-D, picloram, dicamba, MCPA, Ally, and Amber, Wrage says. Use caution as well to prevent drift when spraying pastures, small grain, and other crops adjacent to sunflower.—Tracy Sayler

Weed Info Online

http://www.ag.ndsu.nodak.edu/weeds/

http://plantsci.sdstate.edu/weeds/

http://appliedweeds.coafes.umn.edu/

http://www.oznet.ksu.edu/agronomy/EXTENSION/crops/weed_science.asp

http://www.ianr.unl.edu/pubs/fieldcrops/ec130.htm

2002 Field Survey Pinpoints Weed Problems

A field survey conducted by the National Sunflower Association in eight sunflower-producing states last fall helped pinpoint grass and broadleaf weed problems in sunflower.

Broadleaf weeds with the highest densities across the survey area included kochia and the pigweed species redroot pigweed, waterhemp and Palmer amaranth. Puncturevine was commonly found in Colorado and Kansas. Most common grass weeds across the survey area were green foxtail and volunteer grain.

Kochia, redroot pigweed, wild buckwheat, green foxtail and yellow foxtail were weeds at the highest densities in South Dakota

In North Dakota, kochia generally was the most common broadleaf weed. Other common broadleaves included Canada thistle, marshelder, redroot pigweed, Russian thistle, common ragweed, and wild mustard. As in South Dakota, the foxtail species